U.S. patent number 9,971,149 [Application Number 14/654,076] was granted by the patent office on 2018-05-15 for method for producing a wavefront-corrected optical arrangement comprising at least two optical elements.
This patent grant is currently assigned to Jenoptik Optical Systems GmbH. The grantee listed for this patent is JENOPTIK Optical Systems GmbH. Invention is credited to Markus Augustin, Jan Werschnik.
United States Patent |
9,971,149 |
Werschnik , et al. |
May 15, 2018 |
Method for producing a wavefront-corrected optical arrangement
comprising at least two optical elements
Abstract
The invention relates to a method for producing a
wavefront-corrected optical arrangement comprising at least two
optical elements. Using the method, a total wavefront error in the
optical arrangement is determined and compared to a permissible
tolerance range for the total wavefront error. To perform the
method, the optical elements are individualized by assigning an
individual identifier to each of them, such that individualized
optical elements are obtained, individual surface defects are
measured with correct coordinates on all the individualized optical
elements and the measured individual surface defects are stored
with correct coordinates assigned to the appropriate individualized
optical element. The optical arrangement comprising the
individualized optical elements is produced virtually as a virtual
optical arrangement and a total wavefront error is calculated for
the virtual optical arrangement.
Inventors: |
Werschnik; Jan (Jena,
DE), Augustin; Markus (Jena, DE) |
Applicant: |
Name |
City |
State |
Country |
Type |
JENOPTIK Optical Systems GmbH |
Jena |
N/A |
DE |
|
|
Assignee: |
Jenoptik Optical Systems GmbH
(Jena, DE)
|
Family
ID: |
49886863 |
Appl.
No.: |
14/654,076 |
Filed: |
December 13, 2013 |
PCT
Filed: |
December 13, 2013 |
PCT No.: |
PCT/EP2013/003773 |
371(c)(1),(2),(4) Date: |
June 19, 2015 |
PCT
Pub. No.: |
WO2014/095013 |
PCT
Pub. Date: |
June 26, 2014 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20150346488 A1 |
Dec 3, 2015 |
|
Foreign Application Priority Data
|
|
|
|
|
Dec 20, 2012 [DE] |
|
|
10 2012 112 773 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G06F
30/00 (20200101); G02B 27/0025 (20130101); G02B
27/0012 (20130101); G06F 17/10 (20130101) |
Current International
Class: |
G06G
7/48 (20060101); G02B 27/00 (20060101); G06F
17/50 (20060101); G06F 17/10 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
101968567 |
|
Feb 2011 |
|
CN |
|
102 58 715 |
|
Aug 2004 |
|
DE |
|
10 2005 022 459 |
|
Nov 2006 |
|
DE |
|
0 823 976 |
|
Mar 2002 |
|
EP |
|
WO 2005/121899 |
|
Dec 2005 |
|
WO |
|
Primary Examiner: Thangavelu; Kandasamy
Attorney, Agent or Firm: Muncy, Geissler, Olds & Lowe,
P.C.
Claims
The invention claimed is:
1. A method for producing a wavefront-corrected optical arrangement
comprising at least two optical elements, comprising: in step a)
ascertaining a total wavefront error for the optical arrangement;
in step b) comparing the total wavefront error to a permissible
tolerance range for the total wavefront error; in step c) selecting
at least one of the optical elements if the permissible tolerance
range is exceeded; and in step d) changing the optical properties
of the at least one selected optical element such that the
ascertained total wavefront error for the optical arrangement will
be within the permissible tolerance range, wherein the method
further comprises: before step a): in step a-2) individualizing the
optical elements by assigning them in each case an individual
identifier, such that individualized optical elements are present,
measuring individual surface errors with correct coordinates for
all individualized optical elements, and storing the measured
individual surface errors with the correct coordinates in a manner
in which they are assigned to a respective individualized optical
element, and in step a-1) producing the optical arrangement with
the individualized optical elements virtually as a virtual optical
arrangement, wherein in step a), ascertaining the total wavefront
error of the virtual optical arrangement, by way of calculation,
from the individual surface errors and obtaining a calculated total
wavefront error, wherein the total wavefront error is the totality
of wavefront errors over all field points, such that correction of
the total wavefront error is made possible for all field points
simultaneously; wherein in step b), comparing the calculated total
wavefront error of the virtual optical arrangement to the
permissible tolerance range for the total wavefront error, wherein
in step d) changing the optical properties of an at least one
selected individualized optical element virtually such that the
calculated total wavefront error for the virtual optical
arrangement will be within the permissible tolerance range, wherein
the method further comprises: in step e) storing the virtually
effected changes to the optical properties of the at least one
selected individualized optical element as processing data and made
available such that they can be retrieved repeatedly, and in step
f) processing the at least one selected individualized optical
element according to the processing data, and producing the optical
arrangement with the individualized optical elements.
2. The method as claimed in claim 1, wherein in step a) considering
any desired number of field points, and producing virtual beams for
each field point, and calculating the projection thereof onto each
surface of each individualized optical element while it passes
through the virtual optical arrangement as a subaperture.
3. The method as claimed in claim 2, further comprising: for each
field point, calculating a respective wavefront error from the
individual surface errors of the subaperture and storing the
calculated wavefront error.
4. The method as claimed in claim 3, further comprising: deriving a
contribution to the calculated total wavefront error from each
subaperture.
5. The method as claimed in claim 4, further comprising: adding up
the derived contributions to the calculated total wavefront error
with the correct sign and the correct orientation.
6. The method as claimed in claim 4, further comprising:
decomposing the derived contributions to the calculated total
wavefront error into coefficients, and subsequently adding up the
coefficients with the correct sign and the correct orientation.
7. The method as claimed in claim 1, further comprising: arranging
virtual beams in any desired grid with known grid spacings with
respect to one another.
8. The method as claimed in claim 7, further comprising: selecting
the grid spacings such that the subapertures overlap on the surface
of at least one selected individualized optical element.
9. The method as claimed in claim 1, further comprising: adding a
contribution known as a nominal error of an ideal optical
arrangement to the calculated total wavefront error for each field
point.
10. The method as claimed in claim 1, further comprising:
correcting a distortion in an image plane additionally on a
field-side individualized optical element.
11. The method as claimed in claim 1, wherein the individualized
optical elements are not coated.
12. The method as claimed in claim 11, wherein, in step f) all
uncoated individualized optical elements are transferred into a
finished state before the production of the optical arrangement by
additional steps of coating and mounting on the optical
element.
13. The method as claimed in claim 1, wherein the at least one
selected individualized optical element is not coated.
14. The method as claimed in claim 1, wherein the number of the
individualized optical elements selected in step c) depends on
virtually ascertained total wavefront error.
15. The method as claimed in claim 1, further comprising selecting
combinations of the individualized optical elements from a number
of individualized optical elements on the basis of the measurement
data, wherein the combinations are selected such that the
processing data calculated for a combination is optimized for
manufacture.
Description
This nonprovisional application is a national stage of
International Application No. PCT/EP2013/003773, which was filed on
Dec. 13, 2013, and which claims priority to German Patent
Application No. 10 2012 112 773.5, which was filed in Germany on
Dec. 20, 2012, and which are both herein incorporated by
reference.
BACKGROUND OF THE INVENTION
Field of the Invention
When producing optical arrangements, in particular optical high
performance systems, very strict requirements are imposed on the
tolerances of the entire optical arrangement and the resulting
permissible tolerances of the individual components.
Deviations of individual components from a theoretically ideal
shape and an ideal optical behavior usually manifest in the form of
a wavefront error of the beams that pass through the optical
arrangement. Wavefront errors are two-dimensional or
three-dimensional deviations of the wave fronts (loci of identical
phase) from an ideal wavefront, such as for example an ideal plane
wavefront (plane wave) or an ideal spherical wavefront (spherical
wave).
Description of the Background Art
The manufacture and the use of individual components with narrow
tolerance limits are complex in terms of production, result in many
waste parts, require a high level of outlay in terms of measurement
and are therefore expensive, and it is difficult to produce them in
relatively large numbers.
One possibility for minimizing the production complexity of the
individual components is the introduction of at least two
prefabricated optical compensation elements into the beam path of
an optical arrangement in order to compensate a wavefront
deformation (=wavefront error), as is known for example from EP 0
823 976 B1. To this end, the wavefront error at the optical
arrangement is measured using a wavefront measuring instrument. The
optical compensation elements are chosen in correspondence with the
measurement result. The disadvantage of this solution is the
necessary provision of a number of optical compensation elements,
the necessity for further elements in the optical arrangement, and
the high assembling complexity and space requirement for the
optical compensation elements.
Another approach is known from DE 102 58 715 B4. In the method for
producing an optical arrangement in the form of an optical imaging
system having a multiplicity of optical elements, which is
disclosed therein, first the optical arrangement is assembled. In
this case, the optical elements are arranged in their correct
positions. Subsequently, the assembled optical arrangement is
measured, and a wavefront error in the exit pupil or in a face of
the optical arrangement that is conjugate therewith is ascertained
in a spatially resolved manner. In a next step, at least one
correction area that is provided as a correction asphere at least
one of the optical elements is selected, and a topography and/or a
refractive index distribution of the correction area is calculated,
with which correction of the ascertained wavefront error for the
optical arrangement can be effected. In order to be able to bring
about the necessary changes in correspondence with the calculated
topography and/or the refractive index distribution of the
correction area, the at least one correction asphere is removed
from the optical arrangement and processed in a spatially resolved
manner. It is important here to return the correction asphere after
complete processing to the correct coordinates, i.e. with the
alignment and rotary position that were used in the calculation of
the correction values (topography and/or refractive index
distribution) so as to achieve the desired effect of the correction
of the wavefront error. The compensating element corrects the sum
of the errors of the individual optical elements.
One disadvantage of this procedure is that the optical arrangement
first has to be in fact assembled, then partially disassembled and
finally re-assembled, and if necessary calibrated again. As a
result, additional work steps are necessary to produce the optical
arrangement.
From a solution according to DE 10 2005 022 459 A1, a method for
optimizing the quality of an optical system is known, which
comprises at least two elements with optically effective surfaces
(optical elements). A resulting specific wavefront error is
ascertained from the individual optical elements or from groups of
optical elements by determining deviations of an actual form of the
surface shapes from a predetermined form, and subsequently the
relevant specific wavefront error is calculated by way of computer.
The only disclosed calculation rule is, however, here not suitable
for field-dependent simulation of the specific wavefront errors.
The expected total wavefront error of the optical system is
predetermined by way of computer on the basis of the calculation
results, and it is ascertained by way of which surface shapes the
total wavefront error can be corrected, wherein the ascertained
specific wavefront errors are taken as the basis. These ascertained
surface shapes are formed on at least one surface, and only then is
the optical system actually assembled. According to the procedure
of DE 10 2005 022 459 A1, the wavefront errors are corrected in a
field-independent manner. This approach is not expedient in
field-dependent optical systems (for example projection lenses),
since the wavefront error of a field point requires individual
(=field-point-dependent) correction. According to DE 10 2005 022
459 A1, each wavefront error is corrected at the same wavefront
(phase function), as a result of which the correction potential is
not fully exploited.
SUMMARY OF THE INVENTION
The invention is based on the object of proposing a method with
which efficient production of a wavefront-corrected optical
arrangement is possible.
The object is achieved with a method for producing a
wavefront-corrected optical arrangement comprising at least two
optical elements, in which in a step a) a total wavefront error for
the optical arrangement is ascertained; in a step b) the total
wavefront error is compared to a permissible tolerance range for
the total wavefront error; in a step c) at least one of the optical
elements is selected if the permissible tolerance range is
exceeded, and in a step d) the optical properties of the at least
one selected optical element are changed such that the ascertained
total wavefront error for the optical arrangement will be within
the permissible tolerance range. The method according to the
invention is characterized in that in a step a-2), which is to be
carried out before step a), the optical elements are individualized
by assigning them in each case an individual identifier. The
individual identifier can be, for example, an identifying number, a
combination of letters, numbers and characters, or a barcode on a
mount of the optical element. The identifier can also be present on
a face, for example a circumferential face of the optical element.
A biunique storage position of the respective optical element, for
example in a warehouse, can also be used as an individual
identifier, without the optical element physically containing an
identifier. Individual surface errors are measured with the correct
coordinates for all individualized optical elements, and the
measured individual surface errors are stored with the correct
coordinates in a manner in which they are assigned to the
respective individualized optical element. What is useful here is
for each optical element to receive a mark, from which a specific
rotary position of the optical element can be derived. In further
embodiments of the invention, the identifier can be used as the
mark, the mark can be part of the identifier, or the mark can be
integrated in the identifier. In a step a-1), which is likewise to
be carried out before step a), the optical arrangement with the
individualized optical elements is produced virtually such that a
virtual optical arrangement is produced. In step a) the total
wavefront error of the virtual optical arrangement is ascertained,
by way of calculation, from the individual surface errors and over
all field points. The result is a calculated total wavefront error,
wherein the total wavefront error is the totality of wavefront
errors over all field points, such that correction of the total
wavefront error is made possible for all field points
simultaneously.
In step b) the calculated total wavefront error is compared to the
permissible tolerance range for the total wavefront error. The
optical properties of the at least one selected individualized
optical element are changed virtually in step d) such that the
calculated total wavefront error for the optical arrangement will
be within the permissible tolerance range. The virtually effected
changes to the optical properties of the at least one selected
individualized optical element are stored as processing data in a
step e) and made available such that they can be retrieved
repeatedly. Finally, in a step f), the at least one selected
individualized optical element is processed according to the
processing data, and the optical arrangement is produced with the
individualized optical elements. After step f), the optical
arrangement has been produced and actually physically exists.
Wavefront errors are deviations of an actual wavefront from an
ideal wavefront (for example sphere or plane wave). The wavefront
errors result for example in Seidel aberrations, image field
curvatures, point images and distortions. The latter can be
described as a tilting of the wavefront.
A total wavefront error is understood to mean the totality of
wavefront errors over all field points. The advantage of the method
according to the invention is that a field-dependent simulation is
made possible and the total wavefront error can be optimized
simultaneously for all field points. Field points within the
meaning of the description can also be object points. Here, the
total wavefront error is optimized such that it is within the
permissible tolerance range.
A field (short for field of view) is here understood to mean the
totality of all field points.
A field point is the starting point or the starting angle of a
beam. Field points are positions in the field which can be
indicated by way of coordinates. A total beam is here the
combination of all beams.
The term field-dependent simulation means that the wavefront error
contributions of any one surface are calculated (simulated) in
dependence on the field point. Typically, an optical system has
lenses, in which the beams of different field points pass through
very different volumes (in particular parts of the surface) (what
are known as field-side lenses). In other lenses, referred to as
pupil-side, or optically effective faces, beams from different
field points pass through nearly identical volumes. The volumes
through which beams pass differently must be taken into account in
the simulation. Field-side lenses are therefore particularly
suitable for correcting field-dependent wavefront errors.
The term correction in this description with respect to the method
according to the invention is understood to mean that the total
wavefront error is optimized with regard to a target value
(permissible tolerance range). If the total wavefront error is
greater or smaller than the target value, adequate correction is
necessary so that the total wavefront error once again matches the
target value. With careful execution of the correction, the total
wavefront error is optimized.
Optical elements are not ideal across their extent, but have small
deviations caused by the material and production methods (surface
errors, irregularities). Here, for example, individual regions of
the surface of the optical element in each case contribute
individually to a wavefront error caused by the optical element.
Said individual surface errors can be measured for example by
tactile means, by interferometer or by strip projection.
What is essential for the invention is the knowledge that the total
wavefront error is caused only to a minor extent by the mounts, the
material homogeneity and the coatings of the individual
individualized optical elements. The most significant contribution
to the total wavefront error for the optical arrangement stems from
irregularities on the surface. It is therefore possible to
calculate a total wavefront error for an optical arrangement merely
on the basis of coordinate-correct measurement data of the
individualized optical elements. Coordinate-correct means that the
contributions to the individual wavefront errors are assigned to
the respective regions of the considered optical element and, with
knowledge of the respective rotary position of the optical element,
the spatial arrangement of the regions and the wavefront errors
caused thereby are known. A rotary position can be defined for
example with respect to a fixed reference point and/or a reference
plane (for example a fixed coordinate system in a laboratory or a
production facility).
In a first embodiment of the method according to the invention,
only those individualized optical elements are taken into account
with which the permissible tolerance limits for the individual
wavefront errors are observed. In a further embodiment of the
method according to the invention, individualized optical elements
with which the permissible tolerance limits are not observed are
also used. The use of individualized optical elements which would
otherwise need to be rejected or re-processed is made possible in
an advantageous manner with the method according to the invention
since the total wavefront error is optimized.
The term total wavefront error refers to a target value that is to
be measured during the execution of the method according to the
invention. This can also be obtained, for example after the total
wavefront error is decomposed into Zernike coefficients, by
individual Zernike coefficients. The target value can be changed so
that the total wavefront error can be influenced in a targeted
manner.
A very expedient embodiment of the method according to the
invention is achieved if in step a) a number of virtual beams
(=partial beams of a total beam) are produced and the behavior
thereof when passing through the virtual optical arrangement is
calculated. The starting point of the virtual beams is the object
plane. In step a), any desired number of field points are
considered, and a virtual beam is produced for each field point.
The projection of each virtual beam onto each surface of each
individualized optical element (subaperture) while it passes
through the virtual optical arrangement is calculated.
With a projection of a virtual beam starting from a field point on
each surface of each individualized optical element, an area is
covered which, depending on the position of the individualized
optical element in the beam path of the optical arrangement and the
associated divergence, convergence or parallelism of the rays of
the respective virtual beam at this position, has a specific size,
shape and position relative to the optical arrangement. For each
field point, this specific size, shape and position relative to the
optical arrangement is ascertained and stored in each case as what
is known as a subaperture. That is to say, each field point leaves,
by way of the virtual beam, an individual "footprint" (region) on
the surface of the individualized optical element. Irregularities
on the surface of the individualized optical elements within such a
region (=subaperture) can be converted for example via the
refractive index of the individualized optical element into an
individual wavefront error and stored. Each individual wavefront
error contributes to the calculated total wavefront error.
Any surface and any subaperture can be described in various ways
that are known to a person skilled in the art by using a reference
system. For example, a reference system, for example a reference
plane or a suitable coordinate system, can be defined, with respect
to which the surface or the subaperture can be described uniquely.
Coordinate systems can be, for example, two-dimensional or
three-dimensional Cartesian coordinate systems or polar coordinate
systems. The surface and/or the subaperture can also be described
by function systems. For example, an irregularity or an individual
wavefront error can be described by a Zernike representation using
the Zernike polynomials. At the same time, the coefficients have an
application-oriented relevance. They enable better assessment of
the effect of the individual surfaces or of the subapertures on the
entire optical arrangement. A reference system can be selected in
dependence on the configuration of a respective (individualized)
optical element. A description of the surface is given in a
coordinate-correct and position-correct fashion.
A contribution to the calculated total wavefront error can be
derived from each subaperture. The calculated total wavefront error
is therefore understood to mean, and can be illustrated as, a
resulting error from the number of contributions of the
subapertures. The calculated total wavefront error can therefore be
ascertained by adding up the derived contributions with the correct
sign and correct orientation. In a further embodiment of the method
according to the invention, the derived contributions to the
calculated total wavefront error are decomposed into coefficients,
and subsequently the coefficients are added up with the correct
sign and correct orientation.
The virtual beams can be arranged in any desired grid with
preferably known grid spacings with respect to one another. While
the grid spacings here are preferably known, they do not have to be
identical. The grid can therefore be regular, irregular, or a
combination of regular and irregular grid spacings. The grid can be
in the form of a matrix of field points. It may be useful for
example to select the grid spacings that are situated far away from
the optical axis to be smaller than those that are situated near
the optical axis. As a result, sufficient coverage of the edge
regions of the individualized optical elements by the virtual beams
is therefore advantageously achieved.
Of particular importance here is that the grid spacings are
selected such that the subapertures at least partially overlap on a
surface of the selected individualized optical element. The degree
of overlap can be given in percent for example as a ratio of the
cross-sectional areas of the virtual beams at the position of the
surface of the individualized optical element to the area on which
the virtual rays of at least two virtual beams impinge. Complete
overlap has the advantage that the correction area to be calculated
of the selected individualized optical element does not need to be
interpolated at any point and thus calculation artefacts are
avoided that could possibly result in a falsification of the total
wavefront error between the grid points.
It is furthermore possible to take into account sag and other
deformations of the individualized optical elements, as can occur
for example when mounting the individualized optical elements. If
these are taken into account, the calculation using the method
according to the invention will better match an actual total
wavefront error.
In a development of the method according to the invention, it is
furthermore possible to select from a number of individualized
optical elements and to combine such individualized optical
elements in a respective optical arrangement, with the use and
combination of which necessary changes can be carried out
efficiently in terms of process and manufacturing. On the basis of
the measurement data, combinations of the individualized optical
elements can be selected from a number of individualized optical
elements. The selection of the combinations is effected such that
the processing data calculated for a combination is optimized in
terms of production. For example, it is possible in this way to
produce, measure and individualize larger batches of the optical
elements. From said batches, optimized selection of the
individualized optical elements is possible, with which a maximum
number of optical arrangements for changing the individualized
optical elements can be produced with minimum effort.
In a developing embodiment of the method according to the
invention, a virtual change in the rotary position ("clocking") of
at least one individualized optical element is carried out.
Necessary changes can thereby thus be reduced or be avoided
entirely if advantageously compensatory effects can be used due to
a change in the rotary position. This procedure can be combined
with the above-described selection of individualized optical
elements.
It is additionally possible with the method according to the
invention to correct a distortion in the image plane on a
field-side individualized optical element. The distortion is here
calculated from the local inclination of the field-point-dependent
wavefront error. In addition, the image field curvature can also be
calculated from the total wavefront error at all field points, for
example as field-point-dependent defocus coefficient when
decomposing the total wavefront error into Zernike polynomials.
A contribution to the total wavefront error can also occur with
ideal optical arrangements. This contribution, referred to as a
nominal error, can be taken into account for each field point and
be included in the calculation in step d).
In one embodiment of the method according to the invention, the
individualized optical elements can be bare parts. Bare parts are
individualized optical elements which are not yet coated, for
example not yet provided with an antireflection layer. The bare
parts can be mounted. It is generally known that stresses and
deformations of an optical element can be caused by a mount, which
can additionally cause individual wavefront errors. Individual
optical properties caused by a mount can in step a-2) be measured
and stored separately and/or as a contribution to the individual
optical properties of the respective individualized optical
element.
In one developing embodiment of the method according to the
invention, the at least one selected individualized optical element
is a bare part.
If at least one individualized optical element is a bare part, in
step f) all bare parts are transferred into a finished state before
the actual production of the optical arrangement. This refers to
all finishing steps and processes, by which a bare part is
converted into a fully functional individualized optical element.
This refers in particular to the application of one-layer or
multilayer coatings on and/or the attachment of a mount on the
individualized optical element.
Correction of the total wavefront error on an optical arrangement
having bare parts permits a very flexible selection of the
individualized optical elements or of the surfaces or subapertures
to be changed. This is expedient especially if a dominating
contribution to the total wavefront error is made by what are known
as bare part errors. Bare part errors are for example caused by
irregularities on the surfaces of the bare parts. When the method
according to the invention is executed, each individualized optical
element and any number of individualized optical elements can be
selected. As a result, the tolerance limits permissible for the
bare parts can be increased and thus money can be saved and
throughput times through the manufacturing chain can be
reduced.
If the total wavefront error is corrected largely on the basis of
individualized optical elements which are already coated and
mounted, the selected individualized optical elements should
preferably be determined in advance. They initially remain uncoated
and can be mounted in specific mounts. This somewhat limits the
flexibility of the method according to the invention. However,
errors that result from the mounting or coating can be compensated
for.
When the method according to the invention is executed, the number
of individualized optical elements selected in step c) can be
determined in dependence on the virtually ascertained total
wavefront error. For example, a selection can be made under the
criterion that as few individualized optical elements as possible
must be selected or that the changes necessary for each selected
individualized optical element are kept as small as possible. It is
also possible to define an upper and/or a lower limit for the
number of selected individualized optical elements. When selecting
a relatively large number of individualized optical elements, the
individual tolerances of the selected individualized optical
elements can be selected to be relatively large, such that the
manufacturing complexity for each individualized optical element
decreases. If only one individualized optical element is selected,
it is necessary during the processing thereof to observe
significantly tighter tolerance limits. In a further embodiment, it
is also possible for the individualized optical elements having the
largest contribution to the total wavefront error to be selected
and for its optical properties to be changed so as to keep the
total wavefront error within the permissible tolerance.
By selecting the number of selected individualized optical
elements, it is possible to conduct the method in a flexible
manner. What is expedient, although not necessary, is if the
selected individualized optical elements are bare parts. By
adapting the number of selected individualized optical elements, it
is possible for a tolerance to be observed to be selected such that
it is large, as a result of which manufacturing costs are saved. A
bare part is an optical element which is not (yet) coated. Bare
parts are processing states of optical elements which have not yet
been completely finished.
A change in the optical properties of the at least one selected
individualized optical element is done virtually by treating at
least one surface of the selected individualized optical elements
as a correction area. The correction area can be described by
two-dimensional or three-dimensional coordinates. Further features
can be assigned to the coordinates such that a multidimensional
description of the correction area is possible. The correction area
can also be in the form of a function system, such as for example
using the Zernike functions or Hankel functions, B-splines or NURBS
("Non-uniform rational B-splines"). It is also possible to change
the refractive index locally or to apply or to process diffractive
structures on/into the surface (correction area). Here, the
correction area is measured before and after processing and
compared to a calculated predetermined area. If the correction area
with permissible tolerances corresponds to the predetermined area,
the optical arrangement can be produced. The changes to the optical
properties of the at least one selected individualized optical
element can be carried out on one or on both sides of the selected
individualized optical element.
Owing to the coordinate-correct storing of the individual wavefront
errors of the individualized optical elements, the latter can also
be changed with respect to their rotary position and be calculated
virtually with the changed rotary position in the optical
arrangement. As a result, possible advantageous compensatory
effects can be used which are achieved by a changed rotary position
of one or more individualized optical elements relative to one
another.
It is possible that in the optical arrangement, an individualized
optical element is provided which provides no contribution to the
overall wavefront error per se, i.e. is optically neutral. For
example, a plane plate may be provided in a region of the beam path
of the optical arrangement with parallel beam guidance. At least
one surface of the plane plate is provided as a correction area. If
a correction of the total wavefront error is deemed necessary, the
plane plate is selected and the optical properties thereof are
changed accordingly. The plane plate is then an individualized
optical element of the optical arrangement and optically not
neutral. The advantage here is that all other individualized
optical elements can already have their respective finished state,
for example can have been coated and mounted in a final stage. Only
the optical properties of the selected individualized optical
element are changed. It is also possible for the selected
individualized optical element to not be integrated in the optical
arrangement if a calculated total wavefront error is within
permissible tolerance limits and no correction is necessary.
The object is further achieved by a modification of the method
according to the invention. Here, step f) is provided only as an
optional feature. With this modification, a correction method is
proposed by means of which a wavefront-corrected optical
arrangement can be provided purely virtually.
Processing in step f) can be carried out for example using a local
correction method (zonal correction polishing), IBF (Ion Beam
Figuring), MRF (Magneto Rheological Finishing), FJP (Fluid Jet
Polishing), CCP (Computer-Controlled Polishing), according to the
calculated correction.
Preferably, the method according to the invention is used in
optical arrangements for use with UV radiation in the field of
semiconductor lithography. It can also be used in other fields of
use however and is not limited to imaging optical arrangements.
Several advantages are achieved with the method according to the
invention. Firstly, the construction of optical arrangements that
are difficult to correct is avoided. Owing to the realistic
calculation, optical elements in general and individualized optical
elements specifically can be identified. If further contributions
to the total wavefront error are caused, for example, by mounting,
they are usually of a simple nature (for example astigmatism). The
total wavefront error of the optical arrangement can be optimized
early in the manufacturing process and be carried out individually
for each optical arrangement. In addition, changes to selected
individualized optical elements can be carried out simultaneously.
The throughput time through the manufacturing process is thereby
advantageously shortened. Compared to a pure measurement, the
calculated grid can be selected to be much narrower, as a result of
which artefacts caused by a grid that is too rough are avoided. If
measurements are carried out on very fine grids, a lot of
(measurement) time is necessary for these measurements. The method
according to the invention also permits larger tolerances for the
optical elements, since they can be corrected. As a result, the
rejection rate during production of optical elements is
advantageously reduced. In the method according to the invention,
data such as individual identifiers and surface errors of optical
elements that are stored with the correct coordinates can be used,
which are in many cases either ascertained and stored anyway, or
can be collected with comparatively little effort.
The invention will be explained in more detail below with reference
to figures and exemplary embodiments. In the figures:
Further scope of applicability of the present invention will become
apparent from the detailed description given hereinafter. However,
it should be understood that the detailed description and specific
examples, while indicating preferred embodiments of the invention,
are given by way of illustration only, since various changes and
modifications within the spirit and scope of the invention will
become apparent to those skilled in the art from this detailed
description.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will become more fully understood from the
detailed description given hereinbelow and the accompanying
drawings which are given by way of illustration only, and thus, are
not limitive of the present invention, and wherein:
FIG. 1 shows a simplified illustration of an individualized optical
element,
FIG. 2 shows a schematic illustration of a first exemplary
embodiment of a virtual optical arrangement with individualized
optical elements,
FIG. 3 shows a sequence of schematically illustrated subapertures
of the individualized optical elements of the first exemplary
embodiment, and
FIG. 4 shows a schematic illustration of a second exemplary
embodiment of a virtual optical arrangement with individualized
optical elements having a plane plate.
DETAILED DESCRIPTION
FIG. 1 shows in a simplified manner a lens as an individualized
optical element 2.1 in a semi-perspective illustration. The
individualized optical element 2.1 has a planar circumferential
face which extends coaxially with an optical axis 2.3 running
through the center point of the individualized optical element 2.1.
A lateral surface 2.2 of the optical individualized element 2.1
faces the observer. Provided on the circumferential face of the
individualized optical element 2.1 is a mark 3 in the form of a
point. The mark 3 is applied such that it is offset toward that
lateral surface 2.2 that faces in the direction of an object plane
7.1 in a virtual optical arrangement 1 (see FIGS. 2 and 3).
Likewise provided on the circumferential face is an identifier 9,
by which the individualized optical element 2.1 is individualized
by way of a unique letter/number sequence (here symbolized by
"identifier") being assigned to an optical element 2 and being
noted on the identifier 9.
A first exemplary embodiment of a virtual optical arrangement 1,
illustrated in FIG. 2, has, as essential components, a number of
individualized optical elements 2.1B to 2.1G and a stop 10 along an
optical axis 1.1 of the virtual optical arrangement 1 and an image
plane 6 and an object plane 7.1 in which an object 7 can be
located. The optical axes 2.3 of the individualized optical
elements 2.1B to 2.1G and the optical axis 1.1 of the virtual
optical arrangement 1 coincide. The first exemplary embodiment is
strongly schematized and simplified, and serves only to illustrate
and explain the invention. Every individualized optical element
2.1B to 2.1G has two lateral surfaces, with the lateral surface
facing the object plane 7.1 having the index 1 (B1, C1, . . . G1),
and the lateral surface facing the image plane 6 having the index 2
(B2, C2, . . . G2).
Selected by way of example from any desired number of virtual
beams, a first virtual beam 4 and a second virtual beam 5 are shown
starting from the object 7. The first virtual beam 4 is assigned to
a field point a and the second virtual beam 5 is assigned to a
field point b. The field points a and b are located in a matrix of
field points (see FIG. 3). The first virtual beam 4 starts from the
field point a of the object 7 and propagates along the optical axis
1.1 in the virtual optical arrangement 1. In doing so, the rays of
the first virtual beam 4 diverge. The rays of the first virtual
beam 4 are parallelized by the effect of the individualized optical
elements 2.1B to 2.1D. The first virtual beam 4 has its greatest
cross section across a section of the virtual optical arrangement 1
between the optical elements 2.1D and 2.1E. Owing to the effect of
the individualized optical elements 2.1E, 2.1F and 2.1G, the rays
of the first virtual beam 4 converge again and image the field
point a as the field point a' in the image plane 6. The second
virtual beam 5 propagates starting from the field point b. The
field point b is arranged on the object plane 7.1 next to the point
of intersection between the optical axis 1.1 and the object plane
7.1. The second virtual beam 5 is likewise spread and parallelized
by the individualized optical elements 2.1B to 2.1D and converges
again by way of the individualized optical elements 2.1E, 2.1F and
2.1G before it is imaged as the field point b' in the image plane
6. The lateral surface D1 is provided as a correction area. The
individualized optical element 2.1D is a selected optical element
that is selected from the individualized optical elements 2.1B to
2.1G of the optical arrangement 1 for the correction of a total
wavefront error.
FIG. 3 illustrates a face of the object 7 facing the virtual
optical arrangement 1 and the lateral surfaces B1, C1 and D1 of the
individualized optical elements 2.1B, 2.1C and 2.1D (see FIG. 2) in
plan view. Field points are shown arranged in a matrix on the
object 7. Owing to the matrix, the field points are arranged in a
regular grid with identical grid spacings with respect to one
another. The field points are designated a and b for illustrative
purposes. The first and the second virtual beams 4, 5 are spread
relative to the field points a and b and are imaged as projections
on the lateral surface B1. Said projections have a shape and size
that are determined by the cross section of the virtual beams 4, 5
and by the shape of the lateral surface B1 and are referred to as
subapertures 8. A relative position of each subaperture 8 to the
optical axis 1.1 is determined by the position of the field points
in the matrix and by the position of the respectively considered
lateral surface B1 to G2 of the optical elements B to G in the
optical arrangement. The subaperture 8 of the first virtual beam 4
extends symmetrically about the optical axis 1.1. The subaperture 8
of the second virtual beam 5 is located on the lateral surface B1
in a relative position to the optical axis 1.1, which corresponds
to the relative position of the field point b in the matrix.
The subaperture 8 of the first virtual beam 4 present on the
lateral surface C1 is again symmetrical about the optical axis 1.1
and spread with respect to the subaperture 8 on the lateral surface
B1. The size of the subaperture 8 of the second virtual beam 5 is
likewise increased and the subaperture 8 partially overlaps with
the subaperture 8 of the first virtual beam 4.
The subapertures 8 of the first and second virtual beams 4, 5 are
present on the lateral surface D1 about the optical axis 1.1 and
nearly completely overlap one another. Additionally, a free
surface, i.e. a surface not covered by a mount, of the
individualized optical element 2.1D is nearly completely filled by
the subapertures 8.
A second exemplary embodiment of the optical arrangement 1
according to FIG. 4 corresponds to the first exemplary embodiment,
with the difference that an individualized optical element 2.1H
having lateral surfaces H1 and H2 is arranged between the
individualized optical elements 2.1D and 2.1E next to the stop 10.
The rays of the virtual beams 4, 5, but also the rays of all
virtual beams that are not shown extend parallel to one another
between the individualized optical elements 2.1D and 2.1E. The
individualized optical element 2.1H is configured as a plane plate
made of optical glass. The individualized optical element 2.1H is
neutral in terms of its optical effect.
The method according to the invention will be explained below with
reference to FIGS. 1 to 3. A number of different optical elements 2
is produced. Each optical element 2 is assigned an identifier 9
with which the optical element 2 is individually characterized,
i.e. individualized (FIG. 1). In addition, a mark 3 is applied on
the edge of the optical element 2.1 that is thus individualized.
Starting from the mark 3, every point on the surface of the
individualized optical element 2.1 can be uniquely described by a
suitable coordinate system (Cartesian coordinate system, polar
coordinate system). Every individualized optical element 2.1 is
subsequently measured, and irregularities on the surface of the
individualized optical element 2.1 are captured with the correct
position and coordinate regarding their position, shape and extent
on a surface of the individualized optical element 2.1 and with
respect to the quality of the respective irregularity and the
optical effects that can thus be expected, and are stored, in a
manner in which they are assigned to the individualized optical
element 2.1, in a database (not illustrated) as measurement data. A
position and the coordinates of the individualized optical element
2.1 are captured with respect to the mark 3 which is present on the
individualized optical element 2.1.
In order to virtually produce a virtual optical arrangement 1,
individualized optical elements 2.1B to 2.1G are selected, in
correspondence with the optical elements 2 that are necessary for
production, from the database and the measurement data thereof is
made available. The individualized optical elements 2.1B to 2.1G
are arranged virtually along the optical axis 1.1 with known
extents for the lateral surfaces (position) and with a known rotary
position. The rotary position is given, and known, by the known
position of the mark 3 with respect to a defined reference position
about the optical axis 1.1.
A total wavefront error is ascertained by generating virtual beams
4, 5 in a known grid and with known grid spacings with respect to
one another. The profile of said virtual beams 4, 5 through the
virtual optical arrangement 1 is calculated (FIG. 3). Convergences,
divergences and parallelisms of the virtual beams 4, 5 with respect
to one another are calculated here in addition to the profile.
Since the measurement data and the position and the rotary position
of each individualized optical element 2.1 used in a respective
virtual optical arrangement 1 are known, it is possible to
calculate the virtual beams 4, 5 of the individual wavefront errors
present on a lateral surface B1 to H2 and those present in each
case at a subaperture 8 of an individualized optical element 2.1B
to 2.1G with the correct coordinates by using methods that are
known to a person skilled in the art.
The respective subaperture 8 and/or the lateral surface or surfaces
of each individualized optical elements 2.1 are thus described as
coefficients of Zernike polynomials. By way of using an addition
operation with the correct sign, a total wavefront error for the
virtual optical arrangement 1 is calculated from the individual
wavefront errors. In a further embodiment of the method according
to the invention, a nominal error (residual error of the optical
design) for the virtual optical arrangement 1 is ascertained and
included in the calculation of the total wavefront error.
The lateral surface D2 is established as the correction area from
the start in the described embodiment of the method according to
the invention. The individualized optical element 2.1D is a bare
part, while the other individualized optical elements 2.1B, 2.1C,
2.1E, 2.1F and 2.1G have already been coated and mounted and are in
the final states. After the total wavefront error is calculated, it
is compared to permissible tolerance limits. If the total wavefront
error is outside the permissible tolerance limits, a calculation
that is correct in terms of position and coordinates is carried out
as to what changes need to be carried out regarding the selected
optical element 2.1D, and specifically on the correction area D2,
so as to obtain a total wavefront error that is within the
permissible tolerance limits. The necessary virtual changes with
which the tolerance limits are observed are stored as processing
data.
Subsequently, the selected individualized optical element 2.1D is
processed and changed in correspondence with the processing data.
After processing, the selected individualized optical element 2.1D
is coated and mounted.
The individualized optical elements 2.1B to 2.1G, with which the
virtual optical arrangement 1 was produced, are now used in reality
to produce an optical arrangement and are arranged along an optical
axis 1.1 of the actual optical arrangement (not illustrated) with
the correct position and coordinates. If appropriate, the
individualized optical elements 2.1B to 2.1G are then further
adjusted.
In a further embodiment of the method according to the invention,
the individualized optical element 2.1H in the virtual optical
arrangement 1 according to FIG. 4 is positioned in a section of the
virtual optical arrangement 1, above which the virtual beams nearly
completely overlap and cover the free surface of the optical
elements 2.1D, 2.1E which are arranged there. The individualized
optical element 2.1H theoretically has no optical effect. At least
one of the lateral surfaces H1, H2 is provided as the correction
area. The individualized optical element 2.1H is the selected
individualized optical element, from which measurement data are
likewise captured and stored with the correct position and
coordinates.
After the calculation of the total wavefront error for the virtual
optical arrangement 1, the changes to the correction area or areas
H1 and/or H2 that need to be made are calculated, if necessary, and
stored as processing data, with which a total wavefront error
within permissible tolerance limits is achieved. If no changes are
necessary, it is possible in a further embodiment of the invention
to omit the arrangement of the individualized optical element
2.1H.
In a further embodiment of the method according to the invention, a
contribution by the individualized optical element 2.1H to the
nominal error of the optical arrangement 1 is known and is taken
into account. In further embodiments, at least one further
individualized optical element 2.1 is selected in addition to the
individualized optical element 2.1H.
The invention being thus described, it will be obvious that the
same may be varied in many ways. Such variations are not to be
regarded as a departure from the spirit and scope of the invention,
and all such modifications as would be obvious to one skilled in
the art are to be included within the scope of the following
claims.
* * * * *